The
eyeball and the muscles that control its position. The cornea and the lens focus the light rays onto the back of the eye. The lens regulates the focusing for near and far objects by becoming more or less globular.

3

THE
EYE

The eye has often been compared to a camera. It would be more appropriate
to compare it to a TV camera attached to an automatically tracking tripod—a machine that is self-focusing, adjusts automatically for light intensity,
has a self-cleaning lens, and feeds into a computer with parallel-processing
capabilities so advanced that engineers are only just starting to consider similar
strategies for the hardware they design. The gigantic job of taking the light
that falls on the two retinas and translating it into a meaningful visual scene
is often curiously ignored, as though all we needed in order to see was an image
of the external world perfectly focused on the retina. Although obtaining focused images is no mean task, it is modest compared with the work of the nervous system—the retina plus the brain. As we shall see in this chapter,
the contribution of the retina itself is impressive. By translating light into nerve
signals, it begins the job of extracting from the environment what is useful and
ignoring what is redundant. No human inventions, including computer-assisted
cameras, can begin to rival the eye. This chapter is mainly about the neural
part of the eye—the retina—but I will begin with a short description
of the eyeball,
the apparatus that houses the retina and supplies it with sharp images
of the outside world.

THE
EYEBALL
The collective function of the nonretinal parts of the eye is to keep
a focused, clear image of the outside world anchored on the two retinas.
Each eye is positioned in its socket by the six small extraocular muscles
mentioned in Chapter 2. That there are six for each eye is no accident; they consist
of three pairs, with the muscles in each pair working in opposition, so as to
take care of movements in one of three orthogonal (perpendicular) planes. For both
eyes,
the job of tracking an object has to be done with a precision of a few
minutes of arc—or else we see double. (To see how distressing that can be,
try looking at something and pressing on the side of one eye with your index finger.)
Such precise movements require a collection of finely tuned reflexes, including those that control head position.
The cornea (the transparent front part of the eye) and lens together
form the equivalent of the camera lens. About two-thirds of the bending of light
necessary for focusing takes place at the air-cornea interface, where the
light enters the eye. The lens of the eye supplies the remaining third of the focusing
power,
but its main job is to make the necessary adjustments to focus on objects
at various distances. To focus a camera you change the distance between
lens and film; we focus our eye not by changing the distance between lens and
retina but by changing the shape of the rubbery, jellylike lens—by pulling
or relaxing the tendons that hold it at its margin—so that it goes from
more spherical for near objects to flatter for far ones. A set of radial muscles called
ciliary muscles produces these changes in shape. (When we get older than about
45, the lens becomes hard and we lose our power to focus. It was to circumvent
this major irritation of aging that Benjamin Franklin invented bifocal spectacles.)
The reflex that contracts these ciliary muscles in order to make the
lens rounder depends on visual input and is closely linked to the reflex
controlling the concomitant turning in of the eyes.

Two other sets of muscles change the diameter of the pupil and thus
adjust the amount of light entering the eye, just as the iris diaphragm of
a camera determines the f-stop. One set, with radial fibers like the spokes of
a wheel,
opens the pupil; the other, arranged in circles, closes it. Finally,
the self-cleaning of the front of the cornea is achieved by blinking the lids and
lubricating with tear glands. The cornea is richly supplied with nerves subserving
touch and pain, so that the slightest irritation by dust specks sets up a
reflex that leads to blinking and secreting of more tears.

THE
RETINA
All this intricate superstructure exists in the interests of the retina,
itself an amazing structure. It translates light into nerve signals,
allows us to see under conditions that range from starlight to sunlight, discriminates
wavelength so that we can see colors, and provides a precision sufficient
for us to detect a human hair or speck of dust a few yards away.
The retina is part of the brain, having been sequestered from it early
in development but having kept its connections with the brain proper through
a bundle of fibers—the optic nerve. Like many other structures in
the central nervous system, the retina has the shape of a plate, in this case one
about a quarter millimeter thick. It consists of three layers of nerve-cell
bodies separated by two layers containing synapses made by the axons and dendrites
of these cells.
The tier of cells at the back of the retina contains the light receptors,
the rods and cones. Rods, which are far more numerous than cones, are responsible
for our vision in dim light and are out of commission in bright light. Cones
do not respond to dim light but are responsible for our ability to see fine
detail and for our color vision.
The numbers of rods and cones vary markedly over the surface of the
retina.
In the very center, where our fine-detail vision is best, we have only
cones.
This rod-free area is called thefovea and is about half a millimeter
in diameter.
Cones are present throughout the retina but are most densely packed
in the fovea.
Because the rods and cones are at the back of the retina, the incoming
light has to go through the other two layers in order to stimulate them. We
do not fully understand why the retina develops in this curious backward fashion.
One possible reason is the location behind the receptors of a row of
cells containing a black pigment, melanin (also found in skin). Melanin mops
up the light that has passed through the retina, keeping it from being reflected
back and scattering around inside the eye; it has the same function as the
black paint inside a camera. The melanin-containing cells also help chemically restore
the light-sensitive visual pigment in the receptors after it has been bleached
by light (see Chapter 8). For both functions, the melanin pigment must
be close to the receptors. If the receptors were at the front of the retina, the
pigment cells would have to be between them and the next layer of nerve cells, in
a region already packed with axons, dendrites, and synapses.
As it is, the layers in front of the receptors are fairly transparent
and probably do not blur the image much. In the central one millimeter, however,
where our vision is most acute, the consequences of even slight blurring
would be disastrous, and evolution seems to have gone to some pains to alleviate
it by having the other layers displaced to the side to form a ring of thicker
retina,
exposing the central cones so that they lie at the very front. The resulting shallow pit constitutes the fovea.
Moving from back to front, we come to the middle layer of the retina,
between the rods and cones and the retinal ganglion cells. This layer
contains three types of nerve cells: bipolar cells, horizontal cells, and amacrine
cells.
Bipolar cells receive input from the receptors, as the diagram on this
page shows, and many of them feed directly into the retinal ganglion cells.
Horizontal cells link receptors and bipolar cells by relatively long connections
that run parallel to the retinal layers; similarly, amacrine cells link bipolar
cells and retinal ganglion cells.
The layer of cells at the front of the retina contains the retinal ganglion
cells,
whose axons pass across the surface of the retina, collect in a bundle
at the optic disc, and leave the eye to form the optic nerve. Each eye contains
about 125 million rods and cones but only i million ganglion cells. In the
face of this discrepancy, we need to ask how detailed visual information can be preserved.
Examining the connections between cells in the retina can help resolve
this problem. You can think of the information flow through the retina as
following two paths: a direct path, from light receptors to bipolar cells
to ganglion cells, and an indirect path, in which horizontal cells may be interposed
be-

The
enlarged retina at the right shows the relative positions of the three retinal layers.
Surprisingly, the light has to pass through the ganglion-cell and bipolar-cell layers before it gets to the rods and cones.

An
ophthalmologist looking into the eye would see something like this photograph of a normal retina. The large pale circle is the optic disc; here arteries enter and (darker) veins leave the retina. The darker red pigmented area to the extreme right is the macula; in the center of this region, not shown, is the fovea. The black area at the upper left is normal melanin pigmentation.

Light
enters the eye through the transparent cornea, where much of the bending of light takes place. The white dot in the pupil is a reflection of light.